GCD的栅栏函数

作用：只有当栅栏函数执行完毕后才能执行后面的函数

需求：使用栅栏函数规定线程执行顺序

注意点：栅栏函数不能使用全局并发队列

使用示例

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent*)event

{

    dispatch_queue_t queue = dispatch_queue_create("download", DISPATCH_QUEUE_CONCURRENT);

    //1.异步函数

    dispatch_async(queue, ^{

        for(NSIntegeri =0; i <5; i++){

            NSLog(@"download1 -- %zd -- %@",i,[NSThread currentThread]);

        }

    });

    dispatch_async(queue, ^{

        for(NSIntegeri =0; i <5; i++){

            NSLog(@"download2 -- %zd -- %@",i,[NSThread currentThread]);

        }

    });

    dispatch_barrier_async(queue, ^{

        NSLog(@"+++++++++++++++");

    });

    dispatch_async(queue, ^{

        for(NSIntegeri =0; i <5; i++){

            NSLog(@"download3 -- %zd -- %@",i,[NSThread currentThread]);

        }

    });

    dispatch_barrier_async(queue, ^{

        NSLog(@"+++++++++++++++");

    });

    dispatch_async(queue, ^{

        for(NSIntegeri =0; i <5; i++){

            NSLog(@"download4 -- %zd -- %@",i,[NSThread currentThread]);

        }

    });

}

输出结果：